A mainstream of display devices such as liquid crystal display devices and organic EL display devices is those of an active matrix driving system in which the display devices are driven with a thin film transistor (hereinafter sometimes referred to as TFT) disposed in each pixel. Various insulating layers used in a TFT, such as a gate insulating layer and an interlayer insulating layer, are generally produced by forming a film of an inorganic material such as silicon nitride or silicon oxide by vapor deposition.
The vapor deposition method, however, has a problem that the vacuum equipment required for vapor deposition is very expensive. Furthermore, patterning of the deposited inorganic film requires complicated steps such as application, exposure, and development of a resist material, etching of an inorganic material, and removal of the resist material. Moreover, a flexible display device including a plastic substrate, which has been actively developed in recent years, has problems that cracks tend to occur in a conventional insulating layer made from an inorganic material and the durability of the insulating layer is insufficient.
In order to solve such problems, studies have been made about the use of a photosensitive resin composition as an organic material in a gate insulating layer and an interlayer insulating layer. An insulating layer made from a photosensitive resin composition can reduce the production cost because expensive vacuum deposition equipment is unnecessary. Further, patterning of the insulating layer can be performed by applying, exposing, and developing the photosensitive resin, so that it is possible to process the insulating layer easily without requiring a resist material. Furthermore, since the photosensitive resin layer has higher film toughness than an inorganic film does, crack generation can be suppressed when the layer is used in a flexible display device.
As a photosensitive resin composition for an insulating layer of a TFT, for example, materials containing a polyimide resin are proposed from the viewpoint of heat resistance (Patent Documents 1 and 2).